The separation or removal of liquids, such as water, from gases, such as organic gases, is an important process within the chemical, petrochemical, medical and energy industries. Water removal is important in the primary production of a wide range of organic solvents, in the recovery and recycling of used solvents, and in the removal of water from chemical equilibrium reactions to drive the reaction towards a preferred product.
Another application that requires gas that is free or essentially free of liquids, such as water, is medical breath analysis, which is performed to provide information related to a patient's condition. An example of a gas analysis often performed is capnography, which is the monitoring of respiratory carbon dioxide (CO2) concentration, usually time dependent. The time dependent respiratory CO2 concentration may be used to directly monitor the inhaled and exhaled concentration of CO2, and indirectly monitor the CO2 concentration in a patient's blood. Other gases such as oxygen (O2), carbon monoxide (CO), nitrogen or the like may also be measured individually or in combination.
In breath analysis systems, for example capnography, breath gas can be sampled such as by a mainstream or a sidestream analyzer. In mainstream analyzers, the sample chamber is positioned within the patient's gas stream, usually near the patient's end of the breathing system. This arrangement is normally heavier and more cumbersome than sidestream systems.
In sidestream analyzers, gas is often drawn from the breathing system by a tube. The tube, which may be connected to an adaptor, delivers the gas to a sampling place (such as a sampling chamber). It is preferable that the sampling line is clear of liquids, such as condensed out liquids, in the fluid sample at all times, in order to permit continuous, non-interfered monitoring.
Condensed out liquids generally refer to water that condenses out from the humidity (the water vapor in breath) in the sampling tubes. Condensed out liquids are a major problem commonly hindering breath analyses, particularly sidestream capnography. The internal humidity levels in the tubes are high, especially in proximity to the breath collection area, since the exhaled and inhaled breath is humid and relatively warm. This is also the case in intubated patients who are generally artificially ventilated with gas (for example, air) having up to 100% humidity at a temperature normally above ambient temperature (for example, about 34° C.), depending on the airway humidification system and patient needs. The humidity (water vapors) often condenses on the inside of the tube, particularly as the tube extends farther from the breath collection area due to the temperature decreases.
Various processes that have been used to dehydrate fluids include newer membrane-based techniques such as pervaporation and vapor permeation. Pervaporation is a process that involves a membrane in contact with a fluid (which may include gas and/or liquid) on a feed or upstream side and a vapor on the permeate or downstream side. Usually, a vacuum or an inert gas is applied on the vapor side of the membrane to provide a driving force for the process. Typically, the downstream pressure is lower than the saturation pressure of the permeate. Vapor permeation is quite similar to pervaporation, except that a vapor is contacted on the feed side of the membrane instead of a liquid. As membranes suitable for pervaporation separations are typically also suitable for vapor permeation separations, use of the term “pervaporation” herein encompasses both “pervaporation” and “vapor permeation”.
A variety of different types of membranes have been described for use in pervaporation dehydration processes. The materials used to prepare the membranes include hydrophilic organic polymers such as polyvinylalcohol, polyimides, polyamides, and polyelectrolytes. In addition, inorganic materials such as molecular sieves and minerals (for example, zeolites which are aluminosilicate minerals) having a microporous structure have been used.
Initially, polymer-based pervaporation membranes comprised dense, homogeneous membranes. Typical examples of such membranes are described by Yamasaki et al. [J. Appl. Polym. Sci. 60 (1996) 743-48], which is incorporated herein by reference in its entirety. These membranes suffer from low fluxes (amount of fluid that flows through a unit membrane area per unit time) as they are fairly thick. While the flux of the membranes can be increased by decreasing the thickness of the membranes, this leads to a decrease in mechanical strength and robustness.
Two routes have commonly been used to overcome the problem encountered by the above membranes (some of which may be considered homogeneous membranes). The first route involves the use of an asymmetric membrane in which a dense surface layer is supported on a more porous material made from the same polymer. A typical example of such an asymmetric membrane is disclosed by Huang et al. [Sep. Sci. Tech. 28 (1993) 2035-48], which is incorporated herein by reference in its entirety.
A second route involves the formation of a dense thin film on the surface of a suitable support membrane, wherein the chemical composition of the dense surface layer and the supporting membrane are typically different. Typically, the support membrane is an ultrafiltration membrane that may contain an incorporated fabric to provide additional strength. Examples of these thin film composite membranes are described in U.S. Pat. No. 4,755,299, U.S. Pat. No. 5,334,314, U.S. Pat. No. 4,802,988 and EP 0,381,477. One major disadvantage of these thin-film composite membranes, however, is their fragility. For example, the commonly used cross-linked poly(vinylalcohol) films supported on polyacrylonitrile ultrafiltration membrane supports are readily damaged through the formation of cracks in the films and through parts of the film falling away from the support. Great care must therefore be taken when mounting and using these membranes. It is also difficult to prepare such membranes in such a way that they are free of defects.
A special form of the thin-film composite membranes is referred to as a “Simplex” membrane. These are made up of thin films using alternating layers of oppositely charged polyelectrolytes. The membranes are made by successive immersions in solutions of the two different polylelectrolytes such that a multilayer complex is formed (see for example Krasemann et al. [J. Membr. Sci. 150 (1998) 23-30]; Krasemann et al. [J. Membr. Sci. 181 (2001) 221-8], and Haack et al. [J. Membr. Sci. 184 (2001) 233-43]), which are incorporated herein by reference in their entirety. While a high selectivity and reasonable fluxes can be achieved with the Simplex membranes, these membranes are complex to prepare, as they require multiple coating steps. In order to get ideal performance, up to 60 dipping operations are sometimes needed. Another significant drawback lies in the fact that these membranes cannot tolerate feed water contents higher than 25% without loss of some of the multiple layers.
Nafion® Membrane
A conventionally used way for dehydration of gases by pervaporation is using proton conducting membranes, such as the membranes used in proton exchange membrane fuel cells. These membranes, an example of which is sold under the brand name Nafion® by DuPont, are made of a perfluorinated sulfonic acid polymer. Nafion® membranes, which are fully fluorinated polymers, have high chemical and thermal stability and are stable against chemical attack in strong bases, strong oxidizing and reducing acids, H2O2, Cl, H2 and O2 at temperatures up to about 100° C. Nafion® consists of a fluoropolymer backbone upon which sulfonic acid groups are chemically bonded. However, although usually providing sufficient performance, Nafion® is an expensive material which renders it economically unattractive in most applications.
Nafion® tubes have been used for breath analysis applications (such as capnography), which, as discussed above, require an essentially liquid-free sampled gas. Nafion® tubes include, an inner tube coaxially fitted within the lumen of an outer tube. The inner tube, which is fabricated from a perfluorinated polymer, has a predetermined small internal diameter consistent with breath-by-breath response times. The Nafion® plastic employed exhibits high permeability to moisture (water vapor) but does not readily pass other respiratory gases, such as oxygen and carbon dioxide.
When used in breath analysis, Nafion® is a part of the patient's airway and breath sampling system and thus cannot be transferred from one patient to another and cannot even be re-used for the same patient. This disposable nature of Nafion® increases the cost factor. The cost becomes even more significant in applications that require relatively long Nafion® tubes. For example, when sampling at 150 ml (milliliter)/minute (which is a common flow in capnography, for example), 6 inches of Nafion® are required. This length of Nafion® may cost at least an order of magnitude more than the whole tubing system (such as a breath sampling system).
Because the Nafion® tubing in many applications has very thin walls (typically 0.002-0.003 inch), water permeates through it quickly. The thin walls, however, also dictate a use of secondary structural supports to prevent collapse of the walls. These structural supports complicate the manufacture of the product and also reduce the water permeation.
The Nafion® tubing is fabricated by a process known as blown-film extrusion. This process involves the following steps, which are akin to making trash bags, a material that has walls that are also far too thin to support their own weight. Typical trash bags have a wall thickness of 0.002 or 0.003 inch (hefty trash bags may be a bit thicker). A typical Nafion® medical gas “line” tube is typically 0.0025 inch in wall thickness. And just as a trashcan holds the trash bag open, a mesh insert or an outer sleeve should be used to hold the tubing open from the inside or from the outside, respectively. The mesh may be sufficiently coarsely woven so that it allows circulation of gases to the surface of the tubing; however, as mentioned above, the presence of the mesh insert inside or outside the tube may interfere with the efficiency of the pervaporation process, typically reducing the water pervaporation efficiency by over 50%. Another disadvantage of the Nafion® is the chemically aggressive nature of the raw materials used for its preparation and the difficulty in the processing of these materials. For example, special extrusion means are required in order to allow processing of the Nafion®. Further, integrating Nafion® into tubing systems (such as breath sampling systems) is complicated and require special means.
There is thus a need for membranes and other substrates exhibiting water pervaporation properties, which are effective, easy to handle and manufacture and cost efficient.